In a cellular wireless communications system, a handover or handoff of a user terminal (UT) or handset is a process of transferring an ongoing call or data session from one channel connected to the core network to another channel (e.g., from one cell serviced by one cell tower to another cell serviced by another cell tower). In a mobile satellite communications system, handover is the reassignment or the reconfiguration of radio links between the UT and a satellite, a satellite beam and carrier or triplet frequency set. The criteria to perform handover may vary from system to system, but in general handovers are triggered to select paths with better channel conditions.
Among different types of mobile satellite communications systems, satellites orbiting at different levels above the Earth may be employed, such as low Earth orbit (LEO) satellites, medium Earth orbit (MEO) satellites and geosynchronous Earth orbit (GEO) satellites. A GEO satellite orbits the Earth in a flying in a geosynchronous earth orbit, with an orbital period the same as the Earth's rotation period. A geosynchronous orbit is at an altitude of about 35,786 km (22,236 miles) above the Earth, and typically is directly above the Equator. A LEO satellite typically flies in an orbit around the Earth at an altitude between 160 kilometers or 99 miles (with an orbital period of about 88 minutes) and 2,000 kilometers or 1,200 miles (with an orbital period of about 127 minutes). Further, in a GEO satellite system, the beam footprint on the Earth is generally fixed based on the satellite antenna configuration. Unlike a GEO satellite, a LEO satellite constellation provides a unique mode of operation and associated challenges, where the footprint of each satellite beam on the Earth is constantly moving. Accordingly, mobile satellite systems employing LEO satellites require frequent handovers due to the beam movement across the Earth (even when there is no terminal movement and the channel condition is constant). In addition line of sight between the terminal and the satellite may change due to satellite movement.
What is needed, therefore, are approaches for efficient, dynamic and continuous handover processes, which encompass selection of an optimal path (consisting of a satellite, a satellite beam and carrier frequency set) over which a user terminal (UT) communicates with the radio access network in a mobile satellite communications system.